Effect of SO and SO Exposure to Cu-CHA on Surface Nitrate and NO Formation for NH-SCR.

We report effects of SO and SO exposure on ammonium nitrate (AN) and NO formation in Cu-CHA used for NH-SCR. First-principles calculations and several characterizations (ICP, BET, XRD, UV-vis-DRS) were applied to characterize the Cu-CHA material and speciation of sulfur species. The first-principles calculations demonstrate that the SO exposure results in both (bi)sulfite and (bi)sulfate whereas the SO exposure yields only (bi)sulfate. Furthermore, SOx adsorption on framework-bound dicopper species is shown to be favored with respect to adsorption onto framework-bound monocopper species. Temperature-programmed reduction with H shows two clear reduction states and larger sulfur uptake for the SO-exposed Cu-CHA compared to the SO-exposed counterpart. Temperature-programmed desorption of formed ammonium nitrate (AN) highlights a significant decrease in nitrate storage due to sulfur species interacting with copper sites in the form of ammonium/copper (bi)bisulfite/sulfate. Especially, highly stable sulfur species from SO exposure influence the NO-SCR chemistry by decreasing the NO selectivity during NH-SCR whereas an increased NO selectivity was observed for the SO-exposed Cu-CHA sample. This study provides fundamental insights into how SO and SO affect the NO formation during ammonium nitrate decomposition in NH-SCR applications, which is a very important topic for practical applications.